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Isaac is a rowdy person who enjoys fighting, taking comments from his guild members like "You're really naggy, Commander," and "Chief is an idiot" in stride and treating them more like compliments. He tends to be easily irritated, but is not unwilling to listen to others. He invited Shiroe to join his guild a few months after Debauchery Tea Party was disbanded but got turned down. After forming the Round Table Alliance, he realized that he only understood the surface of Shiroe's personality.[4]

Like many Players during the initial aftermath of the Catastrophe, Isaac continued to treat this new world like it was still a game, such as treating People of the Land like NPCs with no minds of their own, and he bought EXP Pots from Hamelin at the expense of the new Players under Hamelin's thrall. This unbecoming side of him was dropped shortly after the founding of the Round Table Conference even though he continued to treat Landers like NPCs for a short while longer.

He's a nicer person than he is known to be, aiming for his guild to surpass level 90 to keep them from thinking about their current situation and being concerned over Akiba's fate. Soujiro says that Isaac is "just a little shy," although whether that was true or simply to annoy Isaac is unknown. Nyanta, after Isaac and some of his Summoners clean up Log Horizon's new guild building with their Undines' water, says that the Black Sword Knights are actually good people. Despite his initial impressions of Iselus, he quickly came to respect how hard Iselus was working to become a suitable successor to his grandfather. Furthermore, in one of the SNG side quests, a dressed-up Isaac ends up helping a little girl who wanted candy by demanding some from Charasin while in-character.

In battle, Isaac, like Krusty, acts in a way opposite of his normal behavior. Although he is normally hot-blooded and easily agitated, he is methodical and calculating in battle.[5]

Isaac seems to have a strong respect towards the hard-work of others where he showed his strong disapproval towards Eins's suggestion in confiscating the bank account of certain people that exceed a certain limit.

At the beginning of Resolve, Isaac is seen leading his guild as Shiroe talks about the capabilities of guilds.

When Maryelle and Shiroe discuss the situation in Akiba, Maryelle tells him that the Black Sword Knights and the Silver Swords are aiming for level 91. She seems to hold a rather low opinion of Isaac, saying that "the Black Sword Knights always have a thing for being the best," and characterizes him as a person who is "proud, or maybe he just enjoys excluding others" due to his strict requirements for recruiting new members. Shiroe comments that Isaac is trying to compete with the D.D.D, which simply absorbed smaller guilds, by using "quality over quantity." However, this presented another problem—they would have to fight monsters level 85 or above, which is difficult after the Catastrophe.

This brings the two to the main issue at hand: the guild's usage of EXP Pots, acquired from Hamelin through the latter guild's abuse and usage of low-level players that they tricked into joining. This leads to Shiroe's determination to both better the city and to destroy Hamelin.

Isaac is seen enjoying a Crescent Burger he bought from the Crescent Moon Alliance with Rezarick[7]. Later, he is seen after receiving an invitation from Maryelle and Shiroe to attend the Round Table Council meeting. He comments to Rezarick that he is interested in the meeting solely because Shiroe had a hand in the operation.[8]

Isaac and his stats, as shown in Episode 9

Isaac and the other guild masters meet at the Guild Building to attend the Round Table Conference. When Shiroe, as the organizer of the meeting, begins to introduce himself, Isaac identifies him as "Tea Party Shiroe" and tells him to keep his introduction short—thus revealing that the two guild masters have a shared history. After William's departure, Isaac argues with Shiroe over what to do about the current situation in Akiba. After Shiroe reveals that the Guild Building is under his control and states that he wishes to create laws for governing all the inhabitants of Akiba, Isaac demands to know what else Shiroe has planned for his guild Log Horizon. In response, Shiroe simply adjusts his glasses and smiles enigmatically.[1]

Isaac angrily demands for Shiroe to reveal his plan. He calms down after Michitaka and the other two Production guild masters reveal the development of a steam engine, wondering why it was so important. When the subject of People of the Land is brought up, Isaac voices many players' opinion that they were merely NPCs that had no value, but was proven wrong by Maryelle. He later agrees to the foundation of the Round Table Alliance and stands with the other guild masters when they reveal it to the populace. When Log Horizon moves into their new guild building, Isaac pays a visit with some of his summoners. Under the pretense of training them with the large space of the building, they summon Undines and flood the place, cleaning it in the process. They promptly leave afterwards as Nyanta comments that he isn't such a bad person.[9]

When the Round Table Council meets to discuss Eastal's invitation, he once again looks down on the People of the Land as NPCs, only to be corrected by Roderick. He is the one who suggests that a member of the Production guilds attend, and when the three Production Guilds leaders can't agree on who to send, Isaac and Soujiro suggest resolving it with Rock-Paper-Scissors.[10]

During the Libra Festival, the Black Sword Knights and D.D.D are in charge of border patrol, checking on People of the Land's carriages and contents, and making sure that everything runs smoothly. Isaac sits in the middle of the Black Sword Knights' tent, giving orders to his men and taking their reports. While he mentally congratulates himself for being a good leader, Krusty comes to check up on him.

When Krusty sits down next to him, Isaac ponders about Shiroe, revealing his past with the Enchanter. He had observed the Debauchery Tea Party and its disbandment. Shiroe, as one of the few players from the Tea Party that continued playing regularly, joined on some raids as a mercenary. Intrigued by Shiroe's tactical prowess and playing style, Isaac attempted to recruit him to no avail, and gave up. After Krusty gets him to understand Shiroe more, Isaac decides to take a walk around the city since just sitting there was getting boring.

When Shiroe realizes that there was a Person of the Land that was trying to sabotage the festival, he orders Isaac to have his men quell any fights that were turning violent. Later, during Raynesia's dinner party, he is among those in charge of protecting her.

Shiroe, as part of his plan to improve relations between the People of the Land and Akiba, asked Isaac to train the Landers in Maihama to raise their levels, which he agrees to. However, he seems to slightly misunderstand Shiroe and decides that he wants to power level them at the same time. During his meeting with Shiroe and Charasin, he mocks Charasin as a "lolicon" due to the latter's interest in Minori's abilities, and displays his annoyance at Rezarick for calling him "Isaac-kun."

In Maihama, Isaac finds himself bored of training the Lander knights, although he commends their determination and their willingness to keep up with his rigorous training. While he takes a break under a tree, Iselus, Rayneshia's younger brother, finds him and begins bugging him. Rezarick soon joins them, and further compounds Isaac's frustration by teaching Icelus to call the guild master "Isaac-kun." With that, Isaac decides to go back to training the knights.

After training with other members of the Black Sword Knights, Isaac prepares to leave when his horse was startled by an arrow. Realizing who was there, he taunts William Massachusetts by repeating "Round Table Alliance, Round Table Alliance," aware that this was why the other battle guild master had approached him.

Soujiro meets up with Krusty, Isaac, and Michitaka, since the three battle guild leaders were having their weapons repaired by Michitaka. Isaac greets Soujiro as "West Wind" and adds that it was unavoidable to encounter enemies. After the meeting, Soujiro is leaving with Isaac when two members of the Black Sword Knights approach them, telling Isaac that they were there to escort him. After waving off the two members, Isaac suggests having his guild merge with Soujiro's due to the uncertainty of the world around them. Although Soujiro disagrees with Isaac's view of the world, the two members cut in to try and persuade Soujiro otherwise—although not in the way that Isaac would like, as their primary concern was the lack of women in the guild. After Soujiro reiterates his decision not to take up Isaac's offer, Isaac leaves after reminding Soujiro that this made them enemies. After Soujiro's brief run-in with Magus, Magus meets up with Isaac in a cave to deliver EXP Pots.

He is mentioned in the meeting between Maryelle, Ichimonjinosuke Akaneya, Woodstock W, and Soujiro after Hamelin and Magus are brought up. It turns out that Woodstock had already gone to Isaac to try and dissuade him from using the EXP Pots. As Woodstock had expected, the discussion turned sour and Isaac refused to budge on his position. Woodstock bemoans that the guild hadn't changed at all since he had left, which was before the Catastrophe.

When Soujiro announces his plan to kill the PKers, Woodstock goes to confront Isaac again. Irritated that Woodstock had come back, Isaac reminds Woodstock that the latter always demanded things from others when he couldn't do it himself. In the process, Isaac reveals his motivation for having the guild aim to surpass level 90 - he saw it as the best course of action for his guild members, to divert them from the boredom of their current world. Once Woodstock mentions the EXP Pots, he initially defends the usage of in-game items before learning that Hamelin was abusing the younger members whom the EXP Pots were coming from. Although fazed for a second, he resumes defending himself, but Woodstock questions whether Isaac was really okay with Hamelin's actions. At that moment, Soujiro arrives to point out that Isaac really did feel guilty about what he was doing, that he was nice guy who was "just a little shy," and that he cared about Akiba despite putting the guild first in his priorities. Soujiro's constant pestering causes Isaac to snap and he tries to attack Soujiro, who simply dodges it. He finally relents and decides to make their PK-hunting quick, only to yell at Woodstock again because the latter was not coming along.

When the PKers are driven into a dead-end by the West Wind Brigade, Soujiro and Isaac show up to kill them, with Isaac calling them pathetic for resorting to Player Killing. When some of them decide to attack Isaac first, he mocks them for their decision before revealing his sword's special ability, which forced his opponents to focus on him. He then easily slaughters them as Soujiro takes out the rest. Afterwards, he tells Soujiro that he wouldn't help him anymore, but would hunt any PKers that wandered into his guild's hunting grounds, and points at Magus while telling Souji that he would continue buying EXP Pots. He then leaves, allowing Soujiro to confront Magus alone.

Black Sword of Pain: Phantasmal-class weapon from conquering "Rhadamanthys' Throne" raid, it is known as world's fastest phantasmal-class weapon to be obtained due to how little grinding was required. Holds an ability coveted by Guardians where a fixed ratio of damage received is reflected as Hate.

Named Guild Tags: Dog tags made in the shape of the Black Sword by craftsmen in Akiba after the Catastrophe. Members who weren't logged in had one made too, Isaac holds on to these.

Armor of Divine Flame: Phantasmal-class armor that requires both "Fantasy Crimson Metal" and "Tatara Realm's God" quests to be cleared. Has high flame and cold resistance, and sets various basic abilities to a higher level.

Round Table Uniform (円卓会議制服): A uniform designed for the members of the Round Table Alliance after its establishment. Isaac’s has modifications to the collar and the trousers. Although there were various opinion about when to wear the uniform, Isaac is in surprisingly high spirits about wearing it.

Experimental Elbow Piece (実験型肘当): An elbow piece made from Phantasmal-class material that Shiroe's group brought back from Abyssal Shaft. Its full abilities are incomplete, but its reduction of cooldown time has been confirmed. It is a recent innovation of Akiba’s researchers and craftsmen.

Small Chibi Origami Frog* (少し歪な折り蛙) : An origami frog that Iselus gave to Isaac as gifts after learning how to make them from Rezarick. Iselus has now become interested in origami, and the side of his desk is now decorated with his creations as well as some of Isaac's Decent Chibi Origami Frogs.

TL note: originally Small Deformed Folded Frog; changed name slightly to make it more communicable to western audiences